/*
 * Copyright (c) 2007, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.nio.channels;

import java.nio.file.*;
import java.nio.file.attribute.FileAttribute;
import java.nio.file.spi.*;
import java.nio.ByteBuffer;
import java.io.IOException;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutorService;
import java.util.Set;
import java.util.HashSet;
import java.util.Collections;

/**
 * An asynchronous channel for reading, writing, and manipulating a file.
 *
 * <p> An asynchronous file channel is created when a file is opened by invoking
 * one of the {@link #open open} methods defined by this class. The file contains
 * a variable-length sequence of bytes that can be read and written and whose
 * current size can be {@link #size() queried}. The size of the file increases
 * when bytes are written beyond its  current size; the size of the file decreases
 * when it is {@link #truncate truncated}.
 *
 * <p> An asynchronous file channel does not have a <i>current position</i>
 * within the file. Instead, the file position is specified to each read and
 * write method that initiates asynchronous operations. A {@link CompletionHandler}
 * is specified as a parameter and is invoked to consume the result of the I/O
 * operation. This class also defines read and write methods that initiate
 * asynchronous operations, returning a {@link Future} to represent the pending
 * result of the operation. The {@code Future} may be used to check if the
 * operation has completed, wait for its completion, and retrieve the result.
 *
 * <p> In addition to read and write operations, this class defines the
 * following operations: </p>
 *
 * <ul>
 *
 * <li><p> Updates made to a file may be {@link #force <i>forced
 * out</i>} to the underlying storage device, ensuring that data are not
 * lost in the event of a system crash.  </p></li>
 *
 * <li><p> A region of a file may be {@link #lock <i>locked</i>} against
 * access by other programs.  </p></li>
 *
 * </ul>
 *
 * <p> An {@code AsynchronousFileChannel} is associated with a thread pool to
 * which tasks are submitted to handle I/O events and dispatch to completion
 * handlers that consume the results of I/O operations on the channel. The
 * completion handler for an I/O operation initiated on a channel is guaranteed
 * to be invoked by one of the threads in the thread pool (This ensures that the
 * completion handler is run by a thread with the expected <em>identity</em>).
 * Where an I/O operation completes immediately, and the initiating thread is
 * itself a thread in the thread pool, then the completion handler may be invoked
 * directly by the initiating thread. When an {@code AsynchronousFileChannel} is
 * created without specifying a thread pool then the channel is associated with
 * a system-dependent default thread pool that may be shared with other
 * channels. The default thread pool is configured by the system properties
 * defined by the {@link AsynchronousChannelGroup} class.
 *
 * <p> Channels of this type are safe for use by multiple concurrent threads. The
 * {@link Channel#close close} method may be invoked at any time, as specified
 * by the {@link Channel} interface. This causes all outstanding asynchronous
 * operations on the channel to complete with the exception {@link
 * AsynchronousCloseException}. Multiple read and write operations may be
 * outstanding at the same time. When multiple read and write operations are
 * outstanding then the ordering of the I/O operations, and the order that the
 * completion handlers are invoked, is not specified; they are not, in particular,
 * guaranteed to execute in the order that the operations were initiated. The
 * {@link java.nio.ByteBuffer ByteBuffers} used when reading or writing are not
 * safe for use by multiple concurrent I/O operations. Furthermore, after an I/O
 * operation is initiated then care should be taken to ensure that the buffer is
 * not accessed until after the operation has completed.
 *
 * <p> As with {@link FileChannel}, the view of a file provided by an instance of
 * this class is guaranteed to be consistent with other views of the same file
 * provided by other instances in the same program.  The view provided by an
 * instance of this class may or may not, however, be consistent with the views
 * seen by other concurrently-running programs due to caching performed by the
 * underlying operating system and delays induced by network-filesystem protocols.
 * This is true regardless of the language in which these other programs are
 * written, and whether they are running on the same machine or on some other
 * machine.  The exact nature of any such inconsistencies are system-dependent
 * and are therefore unspecified.
 *
 * @since 1.7
 */

public abstract class AsynchronousFileChannel
    implements AsynchronousChannel {

  /**
   * Initializes a new instance of this class.
   */
  protected AsynchronousFileChannel() {
  }

  /**
   * Opens or creates a file for reading and/or writing, returning an
   * asynchronous file channel to access the file.
   *
   * <p> The {@code options} parameter determines how the file is opened.
   * The {@link StandardOpenOption#READ READ} and {@link StandardOpenOption#WRITE
   * WRITE} options determines if the file should be opened for reading and/or
   * writing. If neither option is contained in the array then an existing file
   * is opened for  reading.
   *
   * <p> In addition to {@code READ} and {@code WRITE}, the following options
   * may be present:
   *
   * <table border=1 cellpadding=5 summary="">
   * <tr> <th>Option</th> <th>Description</th> </tr>
   * <tr>
   * <td> {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING} </td>
   * <td> When opening an existing file, the file is first truncated to a
   * size of 0 bytes. This option is ignored when the file is opened only
   * for reading.</td>
   * </tr>
   * <tr>
   * <td> {@link StandardOpenOption#CREATE_NEW CREATE_NEW} </td>
   * <td> If this option is present then a new file is created, failing if
   * the file already exists. When creating a file the check for the
   * existence of the file and the creation of the file if it does not exist
   * is atomic with respect to other file system operations. This option is
   * ignored when the file is opened only for reading. </td>
   * </tr>
   * <tr>
   * <td > {@link StandardOpenOption#CREATE CREATE} </td>
   * <td> If this option is present then an existing file is opened if it
   * exists, otherwise a new file is created. When creating a file the check
   * for the existence of the file and the creation of the file if it does
   * not exist is atomic with respect to other file system operations. This
   * option is ignored if the {@code CREATE_NEW} option is also present or
   * the file is opened only for reading. </td>
   * </tr>
   * <tr>
   * <td > {@link StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} </td>
   * <td> When this option is present then the implementation makes a
   * <em>best effort</em> attempt to delete the file when closed by the
   * the {@link #close close} method. If the {@code close} method is not
   * invoked then a <em>best effort</em> attempt is made to delete the file
   * when the Java virtual machine terminates. </td>
   * </tr>
   * <tr>
   * <td>{@link StandardOpenOption#SPARSE SPARSE} </td>
   * <td> When creating a new file this option is a <em>hint</em> that the
   * new file will be sparse. This option is ignored when not creating
   * a new file. </td>
   * </tr>
   * <tr>
   * <td> {@link StandardOpenOption#SYNC SYNC} </td>
   * <td> Requires that every update to the file's content or metadata be
   * written synchronously to the underlying storage device. (see <a
   * href="../file/package-summary.html#integrity"> Synchronized I/O file
   * integrity</a>). </td>
   * </tr>
   * <tr>
   * <td> {@link StandardOpenOption#DSYNC DSYNC} </td>
   * <td> Requires that every update to the file's content be written
   * synchronously to the underlying storage device. (see <a
   * href="../file/package-summary.html#integrity"> Synchronized I/O file
   * integrity</a>). </td>
   * </tr>
   * </table>
   *
   * <p> An implementation may also support additional options.
   *
   * <p> The {@code executor} parameter is the {@link ExecutorService} to
   * which tasks are submitted to handle I/O events and dispatch completion
   * results for operations initiated on resulting channel.
   * The nature of these tasks is highly implementation specific and so care
   * should be taken when configuring the {@code Executor}. Minimally it
   * should support an unbounded work queue and should not run tasks on the
   * caller thread of the {@link ExecutorService#execute execute} method.
   * Shutting down the executor service while the channel is open results in
   * unspecified behavior.
   *
   * <p> The {@code attrs} parameter is an optional array of file {@link
   * FileAttribute file-attributes} to set atomically when creating the file.
   *
   * <p> The new channel is created by invoking the {@link
   * FileSystemProvider#newFileChannel newFileChannel} method on the
   * provider that created the {@code Path}.
   *
   * @param file The path of the file to open or create
   * @param options Options specifying how the file is opened
   * @param executor The thread pool or {@code null} to associate the channel with the default
   * thread pool
   * @param attrs An optional list of file attributes to set atomically when creating the file
   * @return A new asynchronous file channel
   * @throws IllegalArgumentException If the set contains an invalid combination of options
   * @throws UnsupportedOperationException If the {@code file} is associated with a provider that
   * does not support creating asynchronous file channels, or an unsupported open option is
   * specified, or the array contains an attribute that cannot be set atomically when creating the
   * file
   * @throws IOException If an I/O error occurs
   * @throws SecurityException If a security manager is installed and it denies an unspecified
   * permission required by the implementation. In the case of the default provider, the {@link
   * SecurityManager#checkRead(String)} method is invoked to check read access if the file is opened
   * for reading. The {@link SecurityManager#checkWrite(String)} method is invoked to check write
   * access if the file is opened for writing
   */
  public static AsynchronousFileChannel open(Path file,
      Set<? extends OpenOption> options,
      ExecutorService executor,
      FileAttribute<?>... attrs)
      throws IOException {
    FileSystemProvider provider = file.getFileSystem().provider();
    return provider.newAsynchronousFileChannel(file, options, executor, attrs);
  }

  @SuppressWarnings({"unchecked", "rawtypes"}) // generic array construction
  private static final FileAttribute<?>[] NO_ATTRIBUTES = new FileAttribute[0];

  /**
   * Opens or creates a file for reading and/or writing, returning an
   * asynchronous file channel to access the file.
   *
   * <p> An invocation of this method behaves in exactly the same way as the
   * invocation
   * <pre>
   *     ch.{@link #open(Path, Set, ExecutorService, FileAttribute[])
   *       open}(file, opts, null, new FileAttribute&lt;?&gt;[0]);
   * </pre>
   * where {@code opts} is a {@code Set} containing the options specified to
   * this method.
   *
   * <p> The resulting channel is associated with default thread pool to which
   * tasks are submitted to handle I/O events and dispatch to completion
   * handlers that consume the result of asynchronous operations performed on
   * the resulting channel.
   *
   * @param file The path of the file to open or create
   * @param options Options specifying how the file is opened
   * @return A new asynchronous file channel
   * @throws IllegalArgumentException If the set contains an invalid combination of options
   * @throws UnsupportedOperationException If the {@code file} is associated with a provider that
   * does not support creating file channels, or an unsupported open option is specified
   * @throws IOException If an I/O error occurs
   * @throws SecurityException If a security manager is installed and it denies an unspecified
   * permission required by the implementation. In the case of the default provider, the {@link
   * SecurityManager#checkRead(String)} method is invoked to check read access if the file is opened
   * for reading. The {@link SecurityManager#checkWrite(String)} method is invoked to check write
   * access if the file is opened for writing
   */
  public static AsynchronousFileChannel open(Path file, OpenOption... options)
      throws IOException {
    Set<OpenOption> set = new HashSet<OpenOption>(options.length);
    Collections.addAll(set, options);
    return open(file, set, null, NO_ATTRIBUTES);
  }

  /**
   * Returns the current size of this channel's file.
   *
   * @return The current size of this channel's file, measured in bytes
   * @throws ClosedChannelException If this channel is closed
   * @throws IOException If some other I/O error occurs
   */
  public abstract long size() throws IOException;

  /**
   * Truncates this channel's file to the given size.
   *
   * <p> If the given size is less than the file's current size then the file
   * is truncated, discarding any bytes beyond the new end of the file.  If
   * the given size is greater than or equal to the file's current size then
   * the file is not modified. </p>
   *
   * @param size The new size, a non-negative byte count
   * @return This file channel
   * @throws NonWritableChannelException If this channel was not opened for writing
   * @throws ClosedChannelException If this channel is closed
   * @throws IllegalArgumentException If the new size is negative
   * @throws IOException If some other I/O error occurs
   */
  public abstract AsynchronousFileChannel truncate(long size) throws IOException;

  /**
   * Forces any updates to this channel's file to be written to the storage
   * device that contains it.
   *
   * <p> If this channel's file resides on a local storage device then when
   * this method returns it is guaranteed that all changes made to the file
   * since this channel was created, or since this method was last invoked,
   * will have been written to that device.  This is useful for ensuring that
   * critical information is not lost in the event of a system crash.
   *
   * <p> If the file does not reside on a local device then no such guarantee
   * is made.
   *
   * <p> The {@code metaData} parameter can be used to limit the number of
   * I/O operations that this method is required to perform.  Passing
   * {@code false} for this parameter indicates that only updates to the
   * file's content need be written to storage; passing {@code true}
   * indicates that updates to both the file's content and metadata must be
   * written, which generally requires at least one more I/O operation.
   * Whether this parameter actually has any effect is dependent upon the
   * underlying operating system and is therefore unspecified.
   *
   * <p> Invoking this method may cause an I/O operation to occur even if the
   * channel was only opened for reading.  Some operating systems, for
   * example, maintain a last-access time as part of a file's metadata, and
   * this time is updated whenever the file is read.  Whether or not this is
   * actually done is system-dependent and is therefore unspecified.
   *
   * <p> This method is only guaranteed to force changes that were made to
   * this channel's file via the methods defined in this class.
   *
   * @param metaData If {@code true} then this method is required to force changes to both the
   * file's content and metadata to be written to storage; otherwise, it need only force content
   * changes to be written
   * @throws ClosedChannelException If this channel is closed
   * @throws IOException If some other I/O error occurs
   */
  public abstract void force(boolean metaData) throws IOException;

  /**
   * Acquires a lock on the given region of this channel's file.
   *
   * <p> This method initiates an operation to acquire a lock on the given
   * region of this channel's file. The {@code handler} parameter is a
   * completion handler that is invoked when the lock is acquired (or the
   * operation fails). The result passed to the completion handler is the
   * resulting {@code FileLock}.
   *
   * <p> The region specified by the {@code position} and {@code size}
   * parameters need not be contained within, or even overlap, the actual
   * underlying file.  Lock regions are fixed in size; if a locked region
   * initially contains the end of the file and the file grows beyond the
   * region then the new portion of the file will not be covered by the lock.
   * If a file is expected to grow in size and a lock on the entire file is
   * required then a region starting at zero, and no smaller than the
   * expected maximum size of the file, should be locked.  The two-argument
   * {@link #lock(Object, CompletionHandler)} method simply locks a region
   * of size {@link Long#MAX_VALUE}. If a lock that overlaps the requested
   * region is already held by this Java virtual machine, or this method has
   * been invoked to lock an overlapping region and that operation has not
   * completed, then this method throws {@link OverlappingFileLockException}.
   *
   * <p> Some operating systems do not support a mechanism to acquire a file
   * lock in an asynchronous manner. Consequently an implementation may
   * acquire the file lock in a background thread or from a task executed by
   * a thread in the associated thread pool. If there are many lock operations
   * outstanding then it may consume threads in the Java virtual machine for
   * indefinite periods.
   *
   * <p> Some operating systems do not support shared locks, in which case a
   * request for a shared lock is automatically converted into a request for
   * an exclusive lock.  Whether the newly-acquired lock is shared or
   * exclusive may be tested by invoking the resulting lock object's {@link
   * FileLock#isShared() isShared} method.
   *
   * <p> File locks are held on behalf of the entire Java virtual machine.
   * They are not suitable for controlling access to a file by multiple
   * threads within the same virtual machine.
   *
   * @param <A> The type of the attachment
   * @param position The position at which the locked region is to start; must be non-negative
   * @param size The size of the locked region; must be non-negative, and the sum {@code
   * position}&nbsp;+&nbsp;{@code size} must be non-negative
   * @param shared {@code true} to request a shared lock, in which case this channel must be open
   * for reading (and possibly writing); {@code false} to request an exclusive lock, in which case
   * this channel must be open for writing (and possibly reading)
   * @param attachment The object to attach to the I/O operation; can be {@code null}
   * @param handler The handler for consuming the result
   * @throws OverlappingFileLockException If a lock that overlaps the requested region is already
   * held by this Java virtual machine, or there is already a pending attempt to lock an overlapping
   * region
   * @throws IllegalArgumentException If the preconditions on the parameters do not hold
   * @throws NonReadableChannelException If {@code shared} is true but this channel was not opened
   * for reading
   * @throws NonWritableChannelException If {@code shared} is false but this channel was not opened
   * for writing
   */
  public abstract <A> void lock(long position,
      long size,
      boolean shared,
      A attachment,
      CompletionHandler<FileLock, ? super A> handler);

  /**
   * Acquires an exclusive lock on this channel's file.
   *
   * <p> This method initiates an operation to acquire a lock on the given region of this channel's
   * file. The {@code handler} parameter is a completion handler that is invoked when the lock is
   * acquired (or the operation fails). The result passed to the completion handler is the resulting
   * {@code FileLock}.
   *
   * <p> An invocation of this method of the form {@code ch.lock(att,handler)} behaves in exactly
   * the same way as the invocation
   * <pre>
   *     ch.{@link #lock(long, long, boolean, Object, CompletionHandler) lock}(0L, Long.MAX_VALUE,
   * false, att, handler)
   * </pre>
   *
   * @param <A> The type of the attachment
   * @param attachment The object to attach to the I/O operation; can be {@code null}
   * @param handler The handler for consuming the result
   * @throws OverlappingFileLockException If a lock is already held by this Java virtual machine, or
   * there is already a pending attempt to lock a region
   * @throws NonWritableChannelException If this channel was not opened for writing
   */
  public final <A> void lock(A attachment,
      CompletionHandler<FileLock, ? super A> handler) {
    lock(0L, Long.MAX_VALUE, false, attachment, handler);
  }

  /**
   * Acquires a lock on the given region of this channel's file.
   *
   * <p> This method initiates an operation to acquire a lock on the given
   * region of this channel's file.  The method behaves in exactly the same
   * manner as the {@link #lock(long, long, boolean, Object, CompletionHandler)}
   * method except that instead of specifying a completion handler, this
   * method returns a {@code Future} representing the pending result. The
   * {@code Future}'s {@link Future#get() get} method returns the {@link
   * FileLock} on successful completion.
   *
   * @param position The position at which the locked region is to start; must be non-negative
   * @param size The size of the locked region; must be non-negative, and the sum {@code
   * position}&nbsp;+&nbsp;{@code size} must be non-negative
   * @param shared {@code true} to request a shared lock, in which case this channel must be open
   * for reading (and possibly writing); {@code false} to request an exclusive lock, in which case
   * this channel must be open for writing (and possibly reading)
   * @return a {@code Future} object representing the pending result
   * @throws OverlappingFileLockException If a lock is already held by this Java virtual machine, or
   * there is already a pending attempt to lock a region
   * @throws IllegalArgumentException If the preconditions on the parameters do not hold
   * @throws NonReadableChannelException If {@code shared} is true but this channel was not opened
   * for reading
   * @throws NonWritableChannelException If {@code shared} is false but this channel was not opened
   * for writing
   */
  public abstract Future<FileLock> lock(long position, long size, boolean shared);

  /**
   * Acquires an exclusive lock on this channel's file.
   *
   * <p> This method initiates an operation to acquire an exclusive lock on this
   * channel's file. The method returns a {@code Future} representing the
   * pending result of the operation. The {@code Future}'s {@link Future#get()
   * get} method returns the {@link FileLock} on successful completion.
   *
   * <p> An invocation of this method behaves in exactly the same way as the
   * invocation
   * <pre>
   *     ch.{@link #lock(long, long, boolean) lock}(0L, Long.MAX_VALUE, false)
   * </pre>
   *
   * @return a {@code Future} object representing the pending result
   * @throws OverlappingFileLockException If a lock is already held by this Java virtual machine, or
   * there is already a pending attempt to lock a region
   * @throws NonWritableChannelException If this channel was not opened for writing
   */
  public final Future<FileLock> lock() {
    return lock(0L, Long.MAX_VALUE, false);
  }

  /**
   * Attempts to acquire a lock on the given region of this channel's file.
   *
   * <p> This method does not block. An invocation always returns immediately,
   * either having acquired a lock on the requested region or having failed to
   * do so.  If it fails to acquire a lock because an overlapping lock is held
   * by another program then it returns {@code null}.  If it fails to acquire
   * a lock for any other reason then an appropriate exception is thrown.
   *
   * @param position The position at which the locked region is to start; must be non-negative
   * @param size The size of the locked region; must be non-negative, and the sum {@code
   * position}&nbsp;+&nbsp;{@code size} must be non-negative
   * @param shared {@code true} to request a shared lock, {@code false} to request an exclusive
   * lock
   * @return A lock object representing the newly-acquired lock, or {@code null} if the lock could
   * not be acquired because another program holds an overlapping lock
   * @throws IllegalArgumentException If the preconditions on the parameters do not hold
   * @throws ClosedChannelException If this channel is closed
   * @throws OverlappingFileLockException If a lock that overlaps the requested region is already
   * held by this Java virtual machine, or if another thread is already blocked in this method and
   * is attempting to lock an overlapping region of the same file
   * @throws NonReadableChannelException If {@code shared} is true but this channel was not opened
   * for reading
   * @throws NonWritableChannelException If {@code shared} is false but this channel was not opened
   * for writing
   * @throws IOException If some other I/O error occurs
   * @see #lock(Object, CompletionHandler)
   * @see #lock(long, long, boolean, Object, CompletionHandler)
   * @see #tryLock()
   */
  public abstract FileLock tryLock(long position, long size, boolean shared)
      throws IOException;

  /**
   * Attempts to acquire an exclusive lock on this channel's file.
   *
   * <p> An invocation of this method of the form {@code ch.tryLock()}
   * behaves in exactly the same way as the invocation
   *
   * <pre>
   *     ch.{@link #tryLock(long, long, boolean) tryLock}(0L, Long.MAX_VALUE, false) </pre>
   *
   * @return A lock object representing the newly-acquired lock, or {@code null} if the lock could
   * not be acquired because another program holds an overlapping lock
   * @throws ClosedChannelException If this channel is closed
   * @throws OverlappingFileLockException If a lock that overlaps the requested region is already
   * held by this Java virtual machine, or if another thread is already blocked in this method and
   * is attempting to lock an overlapping region
   * @throws NonWritableChannelException If {@code shared} is false but this channel was not opened
   * for writing
   * @throws IOException If some other I/O error occurs
   * @see #lock(Object, CompletionHandler)
   * @see #lock(long, long, boolean, Object, CompletionHandler)
   * @see #tryLock(long, long, boolean)
   */
  public final FileLock tryLock() throws IOException {
    return tryLock(0L, Long.MAX_VALUE, false);
  }

  /**
   * Reads a sequence of bytes from this channel into the given buffer,
   * starting at the given file position.
   *
   * <p> This method initiates the reading of a sequence of bytes from this
   * channel into the given buffer, starting at the given file position. The
   * result of the read is the number of bytes read or {@code -1} if the given
   * position is greater than or equal to the file's size at the time that the
   * read is attempted.
   *
   * <p> This method works in the same manner as the {@link
   * AsynchronousByteChannel#read(ByteBuffer, Object, CompletionHandler)}
   * method, except that bytes are read starting at the given file position.
   * If the given file position is greater than the file's size at the time
   * that the read is attempted then no bytes are read.
   *
   * @param <A> The type of the attachment
   * @param dst The buffer into which bytes are to be transferred
   * @param position The file position at which the transfer is to begin; must be non-negative
   * @param attachment The object to attach to the I/O operation; can be {@code null}
   * @param handler The handler for consuming the result
   * @throws IllegalArgumentException If the position is negative or the buffer is read-only
   * @throws NonReadableChannelException If this channel was not opened for reading
   */
  public abstract <A> void read(ByteBuffer dst,
      long position,
      A attachment,
      CompletionHandler<Integer, ? super A> handler);

  /**
   * Reads a sequence of bytes from this channel into the given buffer,
   * starting at the given file position.
   *
   * <p> This method initiates the reading of a sequence of bytes from this
   * channel into the given buffer, starting at the given file position. This
   * method returns a {@code Future} representing the pending result of the
   * operation. The {@code Future}'s {@link Future#get() get} method returns
   * the number of bytes read or {@code -1} if the given position is greater
   * than or equal to the file's size at the time that the read is attempted.
   *
   * <p> This method works in the same manner as the {@link
   * AsynchronousByteChannel#read(ByteBuffer)} method, except that bytes are
   * read starting at the given file position. If the given file position is
   * greater than the file's size at the time that the read is attempted then
   * no bytes are read.
   *
   * @param dst The buffer into which bytes are to be transferred
   * @param position The file position at which the transfer is to begin; must be non-negative
   * @return A {@code Future} object representing the pending result
   * @throws IllegalArgumentException If the position is negative or the buffer is read-only
   * @throws NonReadableChannelException If this channel was not opened for reading
   */
  public abstract Future<Integer> read(ByteBuffer dst, long position);

  /**
   * Writes a sequence of bytes to this channel from the given buffer, starting
   * at the given file position.
   *
   * <p> This method works in the same manner as the {@link
   * AsynchronousByteChannel#write(ByteBuffer, Object, CompletionHandler)}
   * method, except that bytes are written starting at the given file position.
   * If the given position is greater than the file's size, at the time that
   * the write is attempted, then the file will be grown to accommodate the new
   * bytes; the values of any bytes between the previous end-of-file and the
   * newly-written bytes are unspecified.
   *
   * @param <A> The type of the attachment
   * @param src The buffer from which bytes are to be transferred
   * @param position The file position at which the transfer is to begin; must be non-negative
   * @param attachment The object to attach to the I/O operation; can be {@code null}
   * @param handler The handler for consuming the result
   * @throws IllegalArgumentException If the position is negative
   * @throws NonWritableChannelException If this channel was not opened for writing
   */
  public abstract <A> void write(ByteBuffer src,
      long position,
      A attachment,
      CompletionHandler<Integer, ? super A> handler);

  /**
   * Writes a sequence of bytes to this channel from the given buffer, starting
   * at the given file position.
   *
   * <p> This method initiates the writing of a sequence of bytes to this
   * channel from the given buffer, starting at the given file position. The
   * method returns a {@code Future} representing the pending result of the
   * write operation. The {@code Future}'s {@link Future#get() get} method
   * returns the number of bytes written.
   *
   * <p> This method works in the same manner as the {@link
   * AsynchronousByteChannel#write(ByteBuffer)} method, except that bytes are
   * written starting at the given file position. If the given position is
   * greater than the file's size, at the time that the write is attempted,
   * then the file will be grown to accommodate the new bytes; the values of
   * any bytes between the previous end-of-file and the newly-written bytes
   * are unspecified.
   *
   * @param src The buffer from which bytes are to be transferred
   * @param position The file position at which the transfer is to begin; must be non-negative
   * @return A {@code Future} object representing the pending result
   * @throws IllegalArgumentException If the position is negative
   * @throws NonWritableChannelException If this channel was not opened for writing
   */
  public abstract Future<Integer> write(ByteBuffer src, long position);
}
